Novak Djokovic says a more aggressive approach is key to his excellent form - but insists he is not invincible.

The Serbian, 24, won a pulsating US Open final at Flushing Meadows to claim a third Grand Slam title of the year.

"Something just clicked," he said. "I'm more aggressive and I have a different approach to the semi-finals and finals of major events."

Rafael Nadal praised Djokovic but warned he may find his achievements in 2011 are "impossible to repeat".

At times the tennis was frightening. We've seen some amazing things in this golden era for men's tennis, but several of the rallies in this final hit new heights. Djokovic and Nadal traded brutal, relentless blows. The rallies were seemingly won, time and time again, only to continue sometimes for upwards of 30 strokes. The superstar of tennis in 2011 has just taken his extraordinary season to a whole new level.

Djokovic beat Nadal 6-2 6-4 6-7 (3-7) 6-1, meaning he has now defeated the Spaniard in six finals this season and has won 64 of his 66 matches in 2011.

The world number one holds the Australian Open, Wimbledon and US Open titles after becoming only the sixth man in the open era to win three of the four majors in the same year.

"I am not invincible," added Djokovic. "I just think a positive attitude keeps you on top of your game when you go on court.

"Of course I want to win more majors and prove things to myself. It would be unbelievable to complete the Grand Slam. I need to win the French Open, but I think it will take time."

Djokovic explained why he felt he had taken such a leap forward in 2011: "Last year I made the final but I could have easily lost in the first round. I overcame that. I needed those tough matches to get confidence and the belief that I could win the majors.

"In the previous years, I hadn't changed my game in any big way and my strokes were still the same and was hitting the shots I wanted to hit.

"But I had difficulties approaching semi-finals and finals. I would wait for players to make mistakes. I didn't have the positive attitude. That has changed now - the 2010 US Open [four-set defeat by Nadal] was the turning point.

"I guess it just clicked in my head. It's just that I'm hitting the shots that I maybe wasn't hitting in the last two, three years now. I'm going for it."

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Nadal happy to have reached final

Djokovic, who replaced Nadal as the top-ranked player earlier this year, praised his opponent: "I've had an amazing year and it keeps going but every time I play Rafa it's a big challenge and I want to congratulate him on a great tournament.

"I wish we have many more tough matches in upcoming years."

Nadal, meanwhile, said he gave his all in the final.

"I'm disappointed but this guy is doing unbelievable things so congratulations Novak and your team," he said.

"What you did this year is probably impossible to repeat.

"I try my best in every moment, these kind of matches are very difficult, it brings your body to the limit. I ran to every ball and fought until the last ball."